The present invention relates to one-component room temperature vulcanizable silicone rubber compositions. More particularly, the present invention relates to a method for improving the application rate and thixotropy properties of one-component room temperature vulcanizable silicone rubber compositions.
Room temperature vulcanizable (RTV) silicone rubber compositions combining the properties of high application rate (i.e., easy to extrude and pump) and good thixotropy (i.e., non-sagging in vertical joints) have a significant advantage in the marketplace. It is therefore desirable to provide a one-component RTV silicone rubber composition having both high application rate and good thixotropy properties.
The present invention is based on the discovery that the use of cyano-functional alkoxy silanes as an in situ treating agent for silica filler in certain alkoxy curing RTV silicone compositions will result in the subsequent sealant composition having a higher application rate and improved thixotropy.
It has been found that the application rate of a one component RTV sealant composition is adversely affected by the presence of excessive amounts of isolated silanol on the surface of silica filler used in making the RTV silicone composition. The presence of the silanol tends to result in silanol-silanol and silanol-methoxy condensation reactions which lower subsequent sealant application rate.
The present invention is based on the discovery that cyano-functional alkoxy silanes will react in situ with the isolated filler silanols to silylate and cap the silica surface, thereby eliminating the undesirable filler-filler and filler-polymer interactions and resulting in sealants having higher application rates. At the same time, the polar N.tbd.C-- groups now present on the filler surface are available to form hydrogen bonding networks which enhance overall sealant thixotropy. This latter function of the cyano-functional alkoxy silanes is particularly important when low viscosity polyalkoxy-terminated polydiorganosiloxane polymers are used.
The use of scavenger compounds in RTV silicone rubber compositions to react with excess hydroxy groups whether from methanol, silanol, or water is known in the art. Reference is made, for example, to U.S. Pat. No. 4,395,526 to White et al. (White); U.S. Pat. No. 4,483,973 and U.S. Pat. No. 4,528,353, both to Lucas et al. (Lucas); and U.S. Pat. No. 4,417,042 (Dziark). The scavengers disclosed in the White and Lucas patents are amine functional alkoxysilane compounds while those disclosed in the Dziark patent are silicone-nitrogen compounds. Although the scavenger compounds disclosed in these references are effective in removing the unwanted silanol groups form the surface of the filler, they do not appear to have an effect on the thixotropy properties of the sealant.
The use of cyano-functional polyalkoxy silanes in one part, alkoxy curing RTV silicone compositions is known in the art. Reference is made, for example, to U.S. Pat. Nos. 4,483,973 and 4,528,353, both to Lucas et al. (Lucas); and to U.S. Pat. No. 4,472,551 to White et al. (White). In the compositions disclosed in the Lucas patents, the cyano-functional polyalkoxy silane is used as a self-bonding, i.e., adhesion promoting, agent. In these compositions, excess hydroxy groups on the filler are removed by an amine functional alkoxysilane scavenger compound and not by the adhesion promoting cyano-functional polyalkoxy silane.
The White patent discloses that a preferred procedure for making the RTV composition therein is to agitate under substantially anhydrous conditions a mixture of a polyalkoxy-terminated organopolysiloxane, filler, and an amount of a polyalkoxysilane sufficient to effect the substantial elimination of hydroxy functional groups in the mixture. Thereafter, the condensation catalyst, crosslinking silane and other ingredients can be added. The White patent discloses the use of an organic scavenger for hydroxy groups.